A synthetic analogue of the active site of Fe-containing nitrile hydratase with carboxamido N and thiolato S as donors: synthesis, structure, and reactivities

As part of our work on models of the iron(III) site of Fe-containing nitrile hydratase, a designed ligand PyPSH(4) with two carboxamide and two thiolate donor groups has been synthesized. Reaction of (Et(4)N)[FeCl(4)] with the deprotonated form of the ligand in DMF affords the mononuclear iron(III) complex (Et(4)N)[Fe(III)(PyPS)] (1) in high yield. The iron(III) center is in a trigonal bipyramidal geometry with two deprotonated carboxamido nitrogens, one pyridine nitrogen, and two thiolato sulfurs as donors. Complex 1 is stable in water and binds a variety of Lewis bases at the sixth site at low temperature to afford green solutions with a band around 700 nm. The iron(III) centers in these six-coordinate species are low-spin and exhibit EPR spectra much like the enzyme. The pK(a) of the water molecule in [Fe(III)(PyPS)(H(2)O)](-) is 6.3 +/- 0.4. The iron(III) site in 1 with ligated carboxamido nitrogens and thiolato sulfurs does not show any affinity toward nitriles. It thus appears that at physiological pH, a metal-bound hydroxide promotes hydration of nitriles nested in close proximity of the iron center in the enzyme. Redox measurements demonstrate that the carboxamido nitrogens prefer Fe(III) to Fe(II) centers. This fact explains the absence of any redox behavior at the iron site in nitrile hydratase. Upon exposure to limited amount of dioxygen, 1 is converted to the bis-sulfinic species. The structure of the more stable O-bonded sulfinato complex (Et(4)N)[Fe(III)(PyP[SO(2)](2))] (2) has been determined. Six-coordinated low-spin cyanide adducts of the S-bonded and the O-bonded sulfinato complexes, namely, Na(2)[Fe(III)(PyP[SO(2)](2))(CN)] (4) and (Et(4)N)(2)[Fe(III)(PyP[SO(2)](2))(CN)] (5), afford green solutions in water and other solvents. The iron(II) complex (Et(4)N)(2)[Fe(II)(PyPS)] (3) has also been isolated and structurally characterized.

Alteration of the aurophilic interactions in trimeric gold(I) compounds through charge transfer. Behavior of solvoluminescent Au(3)(MeN=COMe)(3) in the presence of electron acceptors

The ability of the triangular gold(I) complex, Au(3)(MeN=COMe)(3), which as a solid displays the novel property of solvoluminescence (see: Vickery, J. C.; Olmstead, M. M.; Fung, E. Y.; Balch, A. L. Angew.Chem., Int. Ed. Engl. 1997, 36, 1179) to function as an electron donor has been demonstrated through spectroscopic studies and isolation of crystalline adducts with organic acceptor molecules. Four such adducts with nitro-9-fluorenones have been isolated and subject to single-crystal X-ray diffraction study. These are deep yellow [Au(3)(MeN=COMe)(3)].[2,4,7-trinitro-9-fluorenone], red [Au(3)(MeN=COMe)(3)].[2,4,5,7-tetranitro-9-fluorenone], red [Au(3)(MeN=COEt)(3)](2).[2,7-dinitro-9-fluorenone], and red [Au(3)(MeN=COEt)(3)](2).[2,4,7-trinitro-9-fluorenone]. The solid-state structures of [Au(3)(MeN=COMe)(3)].[2,4,7-trinitro-9-fluorenone] and [Au(3)(MeN=COMe)(3)].[2,4,5,7-tetranitro-9-fluorenone] consist of columns in which the planar gold(I) trimers and the nearly planar nitro-9-fluorenones are interleaved with the gold trimers making face-to-face contact with the nitroaromatic portion of the electron acceptor. Thus the organic acceptors disrupt the aurophilic interactions present in crystalline [Au(3)(MeN=COMe)(3)] itself. However, in [Au(3)(MeN=COEt)(3)](2).[2,7-dinitro-9-fluorenone] and [Au(3)(MeN=COEt)(3)](2).[2,4,7-trinitro-9-fluorenone], aurophilic interactions are found which produce dimers, [Au(3)(MeN=COEt)(3)](2), with nearly trigonal prismatic Au(6) cores. These dimers are interleaved with the nitro-9-fluorenone molecules to again form extended columns in which the components make face-to-face contact. Despite the fact that the gold atoms in [Au(3)(MeN=COMe)(3)] and [Au(3)(MeN=COEt)(3)] are in exposed sites and only two-coordinate, there is no evidence of additional coordination of the nitro-9-fluorenones with gold centers in the crystalline adducts.

A mixed oxidation state, binuclear iron complex containing an unsymmetrically coordinating ligand. A ligand-induced switch in redox behavior

Binuclear [FeIIFeIII(BMDP)(O2CPh)3](BF4) (1) was obtained by treating an acetonitrile solution of the fully reduced [FeIIFeII(BMDP)(O2CPh)(MeOH)1.5(H2O)0.5](BF4)2 with 5 equiv of benzoate and then exposing the mixture to oxygen. Examination of [FeIIFeIII(BMDP)(O2CPh)3](BF4) by X-ray crystallography reveals the localized, mixed oxidation state nature of the cation in the solid state. 1H NMR and magnetic susceptibility data for the new complex are also reported. In the absence of dioxygen and other oxidants, treatment of FeIIFeII(BMDP)(O2CPh)(MeOH)1.5(H2O)0.5](BF4)2 with excess benzoate results in the formation of [FeIIFeII(BMDP)(O2CPh)2](BF4)2, which has also been characterized by X-ray diffraction.

Synthesis and reactions of some heterocyclic azacyanines

The one-step reaction of some amino-substituted heterocycles with diiodomethane to give azacyanines is reported. This useful reaction is of wider application than initially reported and includes the synthesis of new substituted pyrido-, isoquino-, benzimadazo-, and benzothiazoazacyanines 7. Furthermore, treatment of these azacyanines with base generally affects a facile opening of the dihydrotriazinium ring resulting in the formation of new heterocycles 10, 11, and 12, which would be difficult to prepare by other means. This reaction takes an additional direction in the case of halo-substituted azacyanines 7b/c/d where treatment with base gives rise to new interesting derivatives of dipyridotriazines 14b/c/d.

Monomeric and dimeric copper(II) complexes of a novel tripodal peptide ligand: structures stabilized via hydrogen bonding or ligand sharing

The novel tripodal ligand N-(bis(2-pyridyl)methyl)-2-pyridinecarboxamide (Py3AH) affords monomeric and dimeric copper(II) complexes with coordinated carboxamido nitrogens. Although many chloro-bridged dimeric copper(II) complexes are known, [Cu(Py3A)(Cl)] (1) remains monomeric and planar with a pendant pyridine and does not form either a chloro-bridged dimer or the ligand-shared dimeric complex [Cu(Py3A)(Cl)]2 (4) in solvents such as CH3CN. When 1 is dissolved in alcohols, square pyramidal alcohol adducts [Cu(Py3A)(Cl)(CH3OH)] (2) and [Cu(Py3A)(Cl)(C2H5OH)] (3) are readily formed. In 2 and 3, the ROH molecules are bound at axial site of copper(II) and the weak axial binding of the ROH molecule is strengthened by intramolecular hydrogen bonding between ROH and the pendant pyridine nitrogen. Two ligand-shared dimeric species [Cu(Py3A)(Cl)]2 (4) and [Cu(Py3A)]2(ClO4)2 (5) have also been synthesized in which the pendant pyridine of one [Cu(Py3A)] unit completes the coordination sphere of the other [Cu(Py3A)] neighbor. These ligand-shared dimers are obtained in aqueous solutions or in complete absence of chloride in the reaction mixtures.

Synthesis, characterization, and polymerization activity of [bis(4,4'-bis(neophyldimethylsilylmethyl)-2,2'-bipyridyl)copper(I)]+CuBr2- and Implications for copper(I) catalyst structures in atom transfer radical polymerization

A series of 4,4'-disilyl-substituted-2,2'-bipyridine ligands were prepared using a metathesis reaction of the dianion of 4,4'-dimethyl-2,2'-bipyridine with several trialkylsilyl chlorides: 4,4'bis(tert-butyldimethylsilylmethyl)-2,2'-bipyridine (dTBDMSbipy), 4,4'-bis(dimethylthexylsilylmethyl)-2,2'-bipyridine (dTHEXbipy), and 4,4'-bis(neophyldimethylsilylmethyl)-2-2'-bipyridine (dNEObipy). It was observed that the side chain length correlated with the ability of the ligand to form hydrocarbon soluble complexes of copper(I) bromide, with dNEObipy forming the most soluble and easily crystallized complexes. The atom transfer radical polymerization (ATRP) of styrene using dNEObipy as the ligand displayed molecular weight control equivalent to other ATRP systems in which solubilizing ligands, such as 4,4'-di-5-nonyl-2,2'-bipyridine or 4,4'-di-n-heptyl-2,2'-bipyridine, were used. The one-to-one complex of dNEObipy with CuBr was prepared and its crystal structure was determined. The resulting complex had the ionic formulation [(dNEObipy)2Cu]+[CuBr2]- and displayed similar activities in styrene ATRP as the standard 2 dNEObipy/CuBr catalyst system. These and other polymerization results in addition to NMR experiments suggest that the predominant copper(I) species formed in ATRP solutions is the 2-to-1 ligand-to-copper(I) cation, [(dNEObipy)2Cu]+, with either a dihalocuprate or halide counteranion, depending upon the conditions.

Redox characteristics of nickel and palladium complexes of the open-chain tetrapyrrole octaethylbilindione: a biliverdin model

Octaethylbiliverdin, C35H46N4O2, is a linear tetrapyrrole that can exist in coordinated form as the fully reduced trianion (OEB)3-, as the two electron oxidized monoanion, (OEBOx)-, or as the one electron oxidized radical, (OEB.)2-. The three-membered electron-transfer series involving [M(OEB)]n with n = +1, 0, and -1, and M = Ni or Pd has been characterized electrochemically. The most highly oxidized members of these series have been isolated in the form of their diamagnetic triiodide salts, [NiII(OEBOx)]I3 and [PdII(OEBOx)]I3, and characterized spectroscopically and by X-ray diffraction. [NiII(OEBOx)]I3 crystallizes in the orthorhombic space group Pbcn with a = 15.121(3) A, b = 16.777(3) A, and c = 14.628(3) A at 130(2) K with Z = 4. Refinement of 3311 reflections with 209 parameters and no restraints yielded wR2 = 0.136 and R = 0.054 for 2643 reflections with I > 2 sigma(I). The structure involves helical coordination of the linear tetrapyrrole ligand about the nickel with all four nitrogen atoms coordinated to the metal and Ni-N distances of 1.867(5) and 1.879(5) A. The triiodide ion is not coordinated to the nickel but sits over one of the meso carbon atoms of the tetrapyrrole. In the solid state, pairs of [NiII(OEBOx)]+ crystallize about a center of symmetry so that two identical tab/slot hydrogen-bonded arrangements involving the lactam oxygen of one complex and methine and two methylene protons of the adjacent cation. Similar hydrogen-bonded motifs are found in other complexes derived from octaethylbilindione. [PdII(OEBOx)]I3 is isomorphic with the nickel analogue and crystallizes in the orthorhombic space group Pbcn with a = 15.2236(6) A, b = 16.7638(7) A, and c = 14.6289(6) A at 90(2) K with Z = 4. Refinement of 6123 reflections with 209 parameters and no restraints yielded wR2 = 0.094 and R = 0.036 for 4042 reflections with I > 2 sigma(I). The odd electron compound PdII(OEB.) has also been isolated and characterized by single-crystal X-ray diffraction. Black PdII(OEB.) crystallizes in the monoclinic space group I2/a with a = 13.274(3) A, b = 18.655(4) A, c = 14.114(3) A, and beta = 116.00(3) degrees at 140(2) K with Z = 4. Refinement of 2030 reflections with 195 parameters and no restraints yielded wR2 = 0.090 and R = 0.042 for 1715 reflections with I > 2 sigma(I). The structure again involves helical coordination of the linear tetrapyrrole to the palladium through the four pyrrole nitrogen atoms. Reduction of the complex causes a slight elongation of the Pd-N bonds from 1.983(2) and 1.986(2) A in [PdII(OEBOx)]I3 to 2.011(4) and 2.012(4) A in PdII(OEB.).

Co(III) complexes with coordinated carboxamido nitrogens and thiolato sulfurs as models for Co-containing nitrile hydratase and their conversion to the corresponding sulfinato species

Recent spectroscopic data suggest that the Co(III) site in Co-containing nitrile hydratase is ligated to carboxamido nitrogens and thiolato sulfurs and most possibly one or more of the bound thiolates exist as sulfenato and/or sulfinato groups. The absence of any Co(III) complex with such coordination makes it quite difficult to predict the reactivity of this kind of Co(III) site. In this paper, the Co(III) complexes of two designed ligands PyPepSH2 (1) and PyPepRSH2 (2) have been reported. The two complexes, namely, (Et4N)[Co(PyPepS)2] (3) and Na[Co(PyPepRS)2] (4) are the first examples of Co(III) complexes with carboxamido nitrogens and thiolato sulfurs as donors. The average Co(III)-Namido and Co(III)-S distances in these complexes lie in the range 1.90-1.92 and 2.22-2.24 A, respectively. Reaction of H2O2 with both complexes readily affords Na[Co(PyPepSO2)2] (5) and Na[Co(PyPepRSO2)2] (6), species in which the thiolato sulfurs are converted to sulfinato (SO2) groups. Such conversion also occurs when solutions of 3 and 4 are exposed to dioxygen in the presence of activated charcoal. These reactions are clean and the S --> SO2 transformation does not introduce significant changes in the metric parameters of these complexes. The reactivity of 3 and 4 indicates that the bound Cys-sulfurs around the biological Co(III) site could be oxidized to sulfinato groups.

Tetrapyrroles as pi Donors: A Pd(2)(2+) Unit Sandwiched between Two Helical Bilindione-Palladium Moieties

Metalation of octaethylbilindione (H(3)OEB) with palladium(II) acetate produces the novel tetranuclear complex [Pd(4)(OEB)(2)] in which a (Pd(I)(2))(2+) unit is sandwiched between two helical Pd(OEB) units (see picture). Treatment of [Pd(4)(OEB)(2)] with pyridine/ethanol gives the odd-electron helical complex [Pd(OEB)].

Structure of an unusually non-polar hydroxy ether

(1R*,3R*,7S*,9S*)-1,4,4-Trimethyl-9-(tert-butyldimethylsilyloxy)-e xo-11-oxatricyclo[5.3.1.0(2,6)]-undecan-3-ol, C19H36O3Si, Mr = 340.90, orthorhombic, Pbca, a = 10.411 (3), b = 13.275 (3), c = 29.085 (10) A, V = 4019 (2) A3, Z = 8, Dx (130 K) = 1.13 g cm-3, lambda(Mo K alpha) = 0.71069 A, mu = 1.24 cm-1, F(000) = 1504, T = 130 K, R = 0.077 for 1005 observed reflections with I greater than 3 sigma(I). The compound, whose connectivity was not readily apparent from spectroscopic and chemical information alone, possesses a tricyclic framework containing an O-bridged bicyclo[3.2.1]octane fused in a cis-exo manner to a cyclopentanol ring. Most noteworthy is the considerably lower polarity of this compound relative to its opposite cyclopentanol stereoisomer. The close proximity [2.78 (1) A] of the bridging O atom in the [3.2.1] system to the hydroxyl O atom on the five-membered ring is suggestive of an intramolecular hydrogen bond, although the H atom could not be located.

Heterocyclic derivatives of 3-substituted-1,1,1-trifluoro-2-propanones as inhibitors of esterolytic enzymes

A series of (alkylthio)trifluoropropanones containing a heterocyclic moiety was synthesized. The compounds were tested for in vitro inhibition of four hydrolytic enzymes including insect juvenile hormone esterase (JHE), eel acetylcholinesterase (AChE), yeast lipase (LP), and bovine alpha-chymotrypsin. The I50 values ranged from 10(-3) to 10(-7) M. 3-(2-Pyridylthio)-1,1,1-trifluoro-2-propanone was found to be the most potent inhibitor as compared to the other tested heterocyclic analogues with an I50 value of 98 nM against JHE from the fifth-instar larvae of Trichoplusia ni. Results from X-ray crystallography showed that the compound exists in a tetrahedral gem-diol form stabilized by an intramolecular hydrogen bond in the solid state. X-ray crystallography of a less potent inhibitor, 3-(4-pyridylthio)-1,1,1-trifluoro-2- propanone, showed that it also exists in the hydrated form, but it lacks an intramolecular hydrogen bond. These results provide indirect support that trifluoromethyl ketones are transition-state mimic inhibitors of esterases, and the bearing of the results on the transition-state mimic theory is discussed. The I50 values against AChE were in the micromolar range. Compounds containing a imidazolyl, triazolyl, and pyrimidyl moiety showed the highest inhibition of this enzyme. Differential selectivity of inhibition was associated with the bond distances between the nitrogen and the carbonyl group as in the natural substrate, when measured in the molecules in their minimal energy conformations. Inhibition of LP was moderate to weak, when compared to JHE and AChE. None of the tested compounds showed significant inhibition of alpha-chymotrypsin.(ABSTRACT TRUNCATED AT 250 WORDS)